A change in the calibration of a thermocouple can cause the equipment being monitored or controlled to be operated at a temperature different from the temperature indicated. This may cause damage to the equipment being controlled or the product in the case of a process. Currently, the accuracy of an installed thermocouple can be obtained by placing a second thermocouple of known calibration beside the installed thermocouple and comparing the relative indicated temperatures. This is not always easy to do and requires periodic checking to be effective.
The use of two thermocouple elements within a metal sheath is taught by Burley in U.S. Pat. No. 4,909,855, by Thom et al in U.S. Pat. No. 4,778,537 and by Hollander in U.S. Pat. No. 5,111,002, while a temperature indicating device using three thermocouple elements is taught by Petry in U.S. Pat. No. 2,696,118.